1. Field of the Invention
This invention relates to a process for preparing silacyclohexane compounds which serve as a liquid crystal.
2. Description of the Prior Art
The liquid crystal display devices make use of optical anisotropy and dielectric anisotropy of liquid crystal substances. Depending on the mode of display, there are a variety of display systems including those of a twisted nematic type (TN type), a supertwisted nematic type (STN type), a super birefringence type (SBE type), a dynamic scattering type (DS type), a guest/host type, a type of deformation of aligned phase (DAP type), a polymer dispersion type (PD type), and an optical mode interference type (OMI type). The most popular display device is one which is based on the Schadt-Helfrich effect and has a twisted nematic structure.
Although the properties of the liquid substances used in these liquid crystal devices depend, more or less, on the type of display, it is commonly required that the liquid crystal substances have a wide range of liquid crystal working temperatures and that they be stable against moisture, air, light, heat, electric field and the like. Moreover, the liquid crystal substances should desirably be low in viscosity and should ensure a short address time, a low threshold voltage and a high contrast in a cell.
As the liquid crystal display devices have wider utility in recent years, the characteristic properties required for liquid crystal materials become much severer. In addition, those characteristics which have never been required for conventional liquid crystal substances are now expected such as a lower drive voltage, a wider working temperature range which could satisfy the needs for on-vehicle materials and an improvement in low temperature performance.
Under these circumstances, we developed novel silacyclohexane-based liquid crystal compounds which contain a silicon atom in the molecule so that the characteristic properties for use as a liquid crystal substance are improved. These liquid crystal compounds have been proposed, for example, in co-pending U.S. application Ser. Nos. 08/377,961, filed Jan. 25, 1995 and 08/395,706, filed Feb. 28, 1995 (corresponding to European Patent Application Nos. 95101167.5, filed Jan. 27, 1995 and 951029.8.1, filed Mar. 1, 1995 and Korean Patent Application Nos. 95-1701, filed Jan. 28, 1995 and 95-4084, filed Feb. 28, 1995, respectively). These compounds of a trans form are particularly useful as a liquid crystal compound. The compounds obtained at the stage of preparation thereof are in the form of a mixture of cis and trans isomers, which requires separation of the trans isomer from the mixture.